There are many situations involving machinery or apparatus having a shaft which may rotate, in which it is necessary or desirable to monitor the angular displacement of the shaft, either continuously or at specified intervals. Applications for a device for this purpose include under water applications, and while rotary shaft position encoders do exist, most are not designed to operate under water. To use a conventional encoder in such applications requires that it be built into a waterproof housing, which adds size, weight and cost to the device.
The present invention overcomes this disadvantage by providing a rotor having a vane which is configured with an edge having an involute profile, affixed to the rotary shaft. An ultrasonic probe transmits energy waves in the form of ultrasonic pulses to the rotor, which are reflected off of the involute edge back to the probe. The time delay between transmission of the energy pulse and reception of the reflected pulse is directly proportional to the distance of the probe from the rotor or, more specifically, to the portion of the involute edge reflecting the ultrasonic pulse. Since the vane of the rotor includes an involute curve, the change in distance, and therefore the duration of the time delay, will always be proportional to the angular displacement of the shaft.
A method of remote reading of utility meters is disclosed in U.S. Pat. No. 4,500,870 by Krohn et al, which utilizes a light probe reflecting light off of a cam having a curved circumferential surface. As the meter shaft rotates, the circumferential surface of the cam approaches or recedes from the light source, and a photodetector is used to monitor the change in the intensity of light, the square of which is inversely proportional to the distance between the light source and the reflecting surface. However, this system presents significant disadvantages.
First, this device relies on the intensity or amplitude of the reflected signal to determine distance from the reflecting surface. Since the inverse square rule applies to intensity, there is not a linear relationship between the intensity of the reflected light and the distance from the cam.
Second, because the circumferential surface of this device is not necessarily involute, the circumferential surface of the cam is not necessarily (and in general would not be) perpendicular to the optical axis of the light source/receiver. This means that the device must rely on diffuse reflection, otherwise the reflected light would in general not reflect back to the receiver. This further affects the intensity readings by the photodetector, and thus the distance calculation, because the reflecting surface cannot be guaranteed to be uniform and reflect the light evenly. In an underwater application this problem is exacerbated, because the reflecting surface would quickly lose its reflectivity, and non-uniformly. For these reasons it would be particularly difficult to adapt this device for use underwater, where ambient light and murky conditions will also affect the light intensity measured by the photodetector.
The present invention is not subject to these disadvantages, because the distance measurement is calculated from the time delay of the transmitted pulse, which has a linear relationship with distance. This renders the device of the present invention more precise than the Krohn device by orders of magnitude. Furthermore, the use of an involute curve profile in the present invention permits the use of conventional ultrasound technology, which is much more suitable for use in underwater applications but requires that the sound energy be reflected directly back to the probe rather than diffusely scattered by the reflecting surface. The involute profile ensures this, in that the axis of the ultrasonic probe will always be perpendicular to the reflecting surface on the involute edge of the rotor vane.